The present invention relates to a toner end detecting method for an electrophotoraphic copier which allows the copier to operate even when a toner end condition is detected by accident or when a developing device incorporated in the copier is replaced with another storing a toner of the same color while a toner end condition is not detected.
Generally, an electrophotoraphic copier has a photoconductive drum or similar image carrier for electrostatically forming a latent image thereon by imagewise exposure. The latent image is developed by a developing device and then transferred transferred to a recording medium as a toner image. The toner remaining on the drum after the image transfer is removed by a cleaning device. The developing device is made up of a developing section for developing the latent image by supplying a developer to the drum, and a toner supply section for supplying a fresh toner to the developing section. The developer may be implemented as a two-component developer consisting of a black toner and a carrier or of a monocolor toner and a carrier. When the toner supply section runs out of toner, i.e., in a toner end condition, a toner end detecting device detects it and causes a display to inform the user of such a condition. In response, the user fills the toner supply section with a fresh toner or replaces the entire developing device having the toner supply section with a new developing device whose toner supply section is full fresh toner. It is a common practice with the toner end detecting device to use a single toner end detection condition in determining whether or not the toner supply section has run out of toner. The toner end detecting device, therefore, is apt to accidentally detect a toner end condition when the developing ability of the developing device changes due to the change in temperature and/or humidity while the copier is in use. In light of this, there has been proposed a method which selectively uses one of a plurality of toner end detection conditions in determining whether or not the toner supply section has run out of toner.
In a copier of the type described, if the user operates the copier without filling the toner supply section with a toner or without replacing the developing device despite the toner end condition, the toner concentration of the developer stored in the developing section will be excessively lowered to render the image density short or to cause the local omission of an image to occur. Moreover, much carrier deposits on the drum to break a cleaning blade and thereby scratches the surface of the drum. While the copier may be inhibited from operating when a toner end condition is detected, it will be inhibited from doing so even when a toner end is detected by accident due to the rapid consumption of toner as would occur in the event of continuous reproduction of documents whose substantial areas are occupied by images. For this reason, it has been customary to allow an electrophotographic copier to produce at least several ten copies after the power source thereof has been turned on again. It has also been customary to cause, when the developing device is replaced with another storing a toner of different color, a toner color sensing device to sense it and cancel the inhibition.
A maintenance free copier is available which uses a miniature developing device and allows the user of the copier to effect maintenance, as desired. The problem with this kind of copier using a miniature developing device is that when the user does not notice a toner end condition and continuously operates the copier without filling the toner supply section with a toner or replacing the developing device, the toner concentration of the developer stored in the developing section sharply decreases. More specifically, the interval between the detection of the toner end condition and the occurrence of the previously mentioned defective image and the adverse effect on the copier body is extremely short. The copier with such user oriented maintenance has to be protected at least from the adverse influence on the copier body. It is, therefore, difficult to allow the copier to produce several copies after the power source has been turned on again, i.e., it is necessary to inhibit the copier from operating after the detection of a toner end condition. This, again, results in the problem that the copier is unoperable when a toner end detection is detected by accident. When the developing device is replaced with another storing a sufficient amount of toner in the toner supply section thereof, the toner end detecting device does not detect a toner end condition any more, allowing the copier to operate. The copier is also allowed to operate when the developing device is replaced with another storing a toner of different color. However, when the developing device in use and not run out of toner is replaced with another storing a toner of the same color, the copier is inhibited from operating when the developing device in use is removed from the copier body and remains in the inhibited state thereafter. This is especially true with a miniature developing device since it is sometimes used as a key counter.